Screening technology for use in printing halftone images is well known. In general, "halftone" refers to the process of generating a continuous variations of intensity on a printed page using arrays of discrete pixels whose intensity is binary, i.e., on or off. Halftoning is achieved by arraying subsets of the smallest addressable units on a page (i.e., pixels) into a series of units or cells formed by a screen mesh or grid. Pixel density is expressed in terms of dots per inch, while halftone cell density or screen ruling is expressed as lines per inch. A halftone "dot" is formed by selectively printing a number of pixels in a given halftone cell, with the optical density or "darkness" being directly proportional to the number of pixels printed. The gray tones which are available to the printer then are generated by turning on or off a select number of dots within the halftone cell. By switching recorder spots in a halftone cell on or off, a "black" pattern of variable size can be built. This corresponds to the halftone dot. Half toning was initially done with actual photographic screens and now can be done electronically.
Traditional screen printing utilizes printing machines that simulate continuous tone by printing halftone dots on a grid where the number of "on" pixels in the dot corresponds to the continuous tone intensity giving the impression of continuous tone. Color printing using such equipment uses the size grid (screen frequency) for each ink but at a series of angles to give an impression of smoothness and continuous tone. With color printing each color is printed at one of the selected angles. In general, colors are printed at least four different screen angles corresponding respectively to magenta, cyan, yellow and black. These four parameters define screen printing; the screen angle, the screen frequency, dot shape and screen origin. The screen origin is the location of the center of a dot with respect to a corner of a halftone cell. Some color printers are using the same angle and mesh for all the inks but varying the origin to achieve more uniform coverage.
In general, there are two types of screening technology, rational and irrational screening. With a rational screen different colors are spaced at several angles about a common rotational axis. Irrational screening is therefore characterized by screen angles which do not yield mathematically rational tangents. The dot shapes that are typically available include round, elliptical, rounded square dots and Euclidian. A Euclidian dot resembles a black dot in the center at intensities less than 40% and four quarter transparent dots at each corner of a cell for intensities in excess of 60%, with a diamond shape generated for intensities between 40 and 60%. The dot shapes are selected according to the requirements of the reproduction process; requirements which include the printing process, the image content and creating special effects with reticulated screens.
Known imaging systems define the gray levels in 256 steps meaning there is normally a minimum of 16.times.16 recorder spots (pixels) which must be combined in the halftone cell to produce a faithful reproduction. Consequently, to print a screen with 133 lines per inch (LPI) the resolution must be at least 2,200 DPI (e.g., 133.times.16=2128). The spot function (i.e. dot function) generator circuitry shapes the cell "spot" and establishes a threshold above which the imager will enable a pixel so that the recorder dot will be switched on. This means that the screening algorithm of the raster image processor (RIP) must step through the spot function for each recorder spot on the film.
It is well known in the art that quality problems include wrong combinations of screen angle and screen ruling lead to undesirable Moire patterns and color shifts when printing. Moreover, dot shape is important in combination with angle and is frequently responsible for structure in single color separations and breaks in vignettes. As is well known, printing problems develop when a colored pattern is generated using a sequence of superimposed but rotated screens. Each screen is at a select angle. However, for perfect reproduction, all of the halftone dots must be identical and located at identical position. This is possible only if the corners of each halftone cell lie exactly on the recorder grid. The resulting angles are represented by the integral offset of X and Y on the recorder grid which defines irrational tangent.
In the past, precomputerized printing processes had observed that the screens located with 30 degree offsets between colors (e.g. screen at 15, 45, 75 and 105 degrees) were preferable. However, these define irrational tangents which are difficult to reproduce using computer imagers. These angle halftone cells do not match the recorder grid and all halftone cells are consequently different. Therefore, the computer must calculate each halftone cell individually, requiring massive computing power to achieve even reasonable performance.
A recent development is the "super cell" corresponding to a screen tile with a number of half-tone dots. The gray levels of the halftone dots within the tile are controlled by multiple precalculated spot functions. Consequently, all halftone dots in a super cell can have different shapes. The super cells are configured such that only the corners of the super cells match the recorder grid. Super cells allow for larger relative precision and better screen angles and screen frequencies than earlier computer imagers. For example, a 9.times.9 super cell at 2540 DPI can be configured with the irrational number of a tangent of 15 degrees, resulting in a screen angle of 15.0013 degrees.
Those skilled in the art will note that other techniques used by computer imagers such as HQS, accurate screening and balanced screening all are super cell screening algorithms offered by different vendors and differ only in the way that they are implemented. HQS uses angle tiles with halftone dots parallel to the tile edge while balanced screening uses tiles parallel to the recorder grid. The dots within the tile are preangled. Accurate screening combines the two; tiles are angled with the recorder grid in addition to halftone dots that are pre-angled.
All of the prior art imagers are characterized by raster image processors which receives data that is compressed to some extent in order to minimize the amount of data storage needed as well as to reduce processor time. Prior art computer imagers must expand the compressed data to create a bit map of the image. However, it would be advantageous to have a computer imager which processes data in compressed form. The present invention is drawn towards such an imager.